Jay Jaiswal & Adam Bevan Designing Rail Steel Composition and Microstructure to Better Resist Degradation V/T SIC Annual Seminar 2017 Jay Jaiswal & Adam Bevan

Overview Background Overview of Phase 1 Aims and objectives Project achievements Performance of available rail steels Damage susceptibility Microstructural characterisation Economic modelling Conclusions and next steps Rail steel grade selection

Background Previous research has focused on investigating vehicle-track characteristics to reduce wheel-rail forces Less effort has been spent on increasing the materials resistance to the imposed forces EN13674-1 defines rail steels with varying hardness, but it is the microstructure that governs damage resistance Rail manufacturers have also recently developed new steels which provide improved resistance to wear and RCF (e.g. HP335) Further research is required to: Understand the reasons for these improvements Provide guidance on the optimum deployment of rail steels

Aims and objectives Improve the understanding of the response of steel microstructures to the imposed loading conditions Establish features of microstructure that provide maximum resistance to key degradation mechanisms What makes a better rail steel Identification of material characteristics that lead to improved performance Design criteria for next generation of rail steels Development of standardised material tests Guidelines for the optimum deployment of current rail steels in appropriate locations within a rail network to minimise life cycle costs

Phase 1 achievements Methodology for identifying the damage susceptibility of track segments has been developed Development of twin disc facility which better represents damaging wheel-rail forces Collection of material samples for full matrix of rail steels covered by the EN standard (along with some new steels) Controlled testing on scaled twin disc rig and comparison with previous historic test data Detailed microstructural examination of a range of twin disc samples Identification of techniques to assess contribution of microstructural and compositional parameters on resistance to key degradation mechanisms Development of a cost-benefit analysis approach to assess the impact of optimum rail steel grade selection

Increase in hardness and tensile strength Available rail steels Key properties specified in EN13674-1 How are they related to in-service performance? How should they be used for the selection of rail grades? Increase in hardness and tensile strength

Twin disc testing Twin disc test facility at British Steel has been used to test a range of rail samples under controlled conditions Significant amount of historic data on the RCF and wear resistance for a wide range of steels analysed

Performance of rail steels EN13674-1 lists a total of 9 rail steel grades which broadly fall into two categories: As-rolled: derive their strength and hardness from the steel composition Heat treated: derive their strength from steel composition and the heat treatment process Results from recent and historic experimental testing was assessed to understand the performance of current rail steels Despite lower hardness of HP335 wear resistance is similar to harder grades HP335 rail shows greater RCF resistance than EN grades with equivalent hardness

Damage susceptibility Rate of rail degradation (and life) is not uniform throughout any railway network Governed by a combination of track, traffic and operating characteristics in addition to the metallurgical attributes of the steel A network is made up of individual segments with varying track characteristics, degradation rates and expected life Selection of rail steel grade to maximise life needs to combine knowledge of: Metallurgical attributes of the available rail steels and Conditions of wheel-rail and vehicle-track interfaces

Route segmentation Routes segmented into sub-assets based on curve radius Susceptibility to the known degradation mechanisms determined for each segment Additional simulation cases undertaken using generic model running over a range of curve radii and cant deficiencies

Damage susceptibility model Input data: Track geometry data Traffic mix Wheel-rail profiles Vehicle models Damage accumulated across the rail head Vampire route simulations Wheel-rail contact forces Calculate wear and RCF damage Detailed representation of duty conditions Peak damage (wear & RCF) for each track segment Divide route into track segments based on curvature and cant deficiency Determine mean and max. for each track section

Damage susceptibility map Rolling Contact Fatigue Wear

Damage susceptibility map Rolling Contact Fatigue Wear

Damage susceptibility criteria Rolling Contact Fatigue Wear Moderate Low High Moderate Low High Low

Damage susceptibility criteria Tracks sections susceptibility to damage categorised based on curve radius In reality this may be influenced by other factors Suggests that with R260 grade steel ~ 35 % of the network is susceptible to high-moderate RCF and wear damage

Microstructural characterisation All rail steels in EN are pearlitic with increase in hardness achieved through enrichment of composition and/or heat treatment (faster rate of cooling) Pearlite is a 3D entity characterised by many parameters – challenge is to identify the influence of these parameters on known degradation mechanisms A range of characterisation techniques employed to determine influence of Heat treatment (normal or accelerated cooling) Hardness Interlamellar spacing Cementite volume fractions Ferrite lattice Neural network analysis to better identify contribution of compositional and microstructural parameters on wear and RCF resistance

Microstructural characterisation Influence of Key Metallurgical Parameters Increased hardness through accelerated cooling of HE Steels beneficial – further benefits from alloying Depth of fragmentation and dissolution of pearlitic cementite – a key factor for RCF resistance Influence of alloying: Silicon increases resistance to cementite dissolution - greater RCF resistance Vanadium alloying imparts better resistance to plastic deformation Manganese addition beneficial for RCF resistance but not Chromium Finer interlamellar spacing considered to have a second order influence Influence of volume fraction cementite not substantiated yet – further data required Influence of other microstructural parameters and decarburisation remains to be established

Economic assessment Quantify and compare costs of using standard (R260) and premium (HP335) rail steels VTISM modelling undertaken on 4 selected routes A number of scenarios investigated including: Baseline ~ R260 rail steel and current maintenance actions Optimal selection ~ HP335 rail steel installed on curves based on damage susceptibility Full implementation ~ HP335 rail steel installed in every curve RCF and wear damage rates for HP335 rail steel reduced based on experimental testing Grinding interval increased for HP335 rail steel (~ 45 MGT) Lower damage depth ≈ less metal removal required during grinding

Cost modelling approach Additional criteria added to reflect reduction in grinding with HP rail VTISM v2.6 does not support updating of RCF / wear rates following re-railing with HP rail v2.7 now includes functionality to define RCF / wear rates by rail type Unit costs updated for HP rail following discussions with NR / British Steel Additional criteria defined in T-SPA to select appropriate rail steel grade prior to maintenance activity (using Flexible Actions Feature) Further validation required

CBA Results Results show a positive net benefit for Infrastructure Managers (IM) from optimum deployment of HP335 rail Cost savings are largely driven by reductions in the maintenance activities (e.g. grinding and renewals) Reduction in grinding effort results in 30 – 40 % reduction in rail grinding costs (as well as increasing rail life)

Conclusion and next steps Project has made some key breakthroughs in understanding the influence of alloying elements and hardness on degradation of rail steel microstructures Damage susceptibility of track sections has been assessed to formulate guidelines for deployment of different rail steels Twin-disc test facility has been developed for future testing of rail steels under more realistic contact conditions Further work proposed to undertake controlled testing and microstructural assessment of a wider range of steels for plain line and S&C as well as degradation at welds and weld repairs

Application of HP rail steels Demand per region for standard and HP rail steel for last 2-years reviewed showing large variation in use of HP rail steel Highlights the need for clear guidance on selection of optimum rail steel grade

Application of HP rail steels To reduce whole life costs, premium rail steels should be considered for use in critical curves where RCF or wear causes the premature replacement of the rail Rolling Contact Fatigue Wear Low High Moderate Moderate Low High Low Used in moderate curves to preserve the ground rail profile and increase the resistance to RCF Used in in tight radius curves with a high wear rate

Acknowledgements Research financed under EPSRC/DfT/RSSB grant EP/M023303/1: ‘Designing steel composition and microstructure to better resist degradation during wheel-rail contact’ In collaboration with: University of Cambridge University of Leeds Cranfield University British Steel Network Rail